The migration or diffusion of small molecules through a solid polymeric medium has been intensively studied and characterised, because it forms the underlying principle of a number of important commercial applications. For example, containment of liquids and gases either in static reservoirs and tanks, or during transport along tubing and pipes clearly relies on very slow or zero diffusion by the mobile material through the walls of the plastic housing. Similarly for the purposes of packaging and shelf-life of materials, from food, drink and pharmaceuticals to commercial goods such as chemicals and electronics, plastic materials are required for packaging in which diffusion of specific gases or liquids is extremely slow. In the case of separation or purification using membranes the differential diffusion or transport of several gases or liquids through a solid membrane material is again the basis of permeation technology.
The phenomenon of diffusion of small molecules through a polymer medium also operates when issues of contamination arise. For example the migration of residual monomer out of a formed polymeric article can often cause unwanted contamination (A. R. Berens, C. A. Daniels, Polym. Eng. Sci., 16, 552 (1976)). Thus, industrial process steps have been developed which extract unreacted monomer from plastic before its use in final fabrication (V. T. Stannet, Polym. Eng. Sci., 18, 1129 (1978)). Similarly, if the polymer manufacturing process involves polymerisation or treatment in solution, traces of residual solvent, which remains occluded in the plastic will leach out of the material over time (T. J. Stanley, M. M. Alger, Ind. Eng. Chem. Res., 28, 865 (1989). The third, common low molecular weight material which is often found to migrate out of a commercial polymer, or article fabricated from it, is a plasticiser. In contrast to the former examples, a plasticiser is intentionally added or compounded into the plastic in order to modify its mechanical properties. However, it too will diffuse out of the polymer with time and present similar issues around contamination. For such systems, of which plasticised PVC is a familiar example, routes to improvement involve the development of new plasticisers (which are less mobile in the polymeric environment or more benign as a contaminant) or the use of physical barriers, impermeable coatings which prevent loss of the plasticisers from the plastic to its environment (A. Jayakrishnan et al., J. Appl. Polym. Sci., 56, 1187 (1995)).
Polyester materials represent an unusual example of a system which can contain low molecular weight molecules. These species are believed to originate from and exist in equilibrium with the parent polymer at temperatures above its melting range (Tm). As a consequence they originate at the manufacturing stage of the polymer. The low molecular weight material is predominantly a cyclic oligomer (trimer) and has a chemical structure identical to that of the high polymer. Its regular structure allows the cyclic oligomer to crystallise easily, thus under certain thermal treatments the material will diffuse to the surface of a solid polyester and crystallise (S. Reichlmaier et al., J. Vac. Sci. Tech., A13, 1217 (1995); and Y. Kawahara et al., Macromol. mater. Eng., 291, 11 (2006)). When this behaviour interferes with the performance of the polyester or its environment, the presence of the cyclic oligomers is seen as undesirable contamination, as a problem which requires a solution. Two approaches have been described to prevent the migration of a polyester cyclic oligomer to the surface of a polyester article either during further processing or in final service. In the first case, articles such as film or fibre are produced from a polyester raw material with a reduced initial content of cyclic oligomer. (Kawahara ibid; U.S. Pat. Nos. 6,020,056; and 6,054,224) This is successful when the fibre or film manufacturing process subjects the polyester to melt temperatures for a duration which is too short to allow the equilibrium level of oligomer to re-establish in the system. The second case involves laminating or coextruding an outer layer of plastic to the article which operates as a barrier to diffusion by the cyclic species to the new surface. This is successful if the laminated surface has properties which match or surpass those of the original polyester but often carries the disadvantage of higher cost (U.S. Pat. No. 5,545,364). To date, however, the use of a coating technology has not been previously applied to perform this function on polyester articles.
The polyester article of particular interest in the present application is film used in the field of flexible electronic or opto-electronic technology, as disclosed in, for instance, WO-A-03/022575. In particular, the film is a substrate on which electronic circuitry is manufactured and mounted in order to drive the electronic operation of the flexible device. The component which comprises the flexible substrate and circuitry is often described as a backplane. During the course of fabrication of the final backplane, the substrate is often exposed to conditions of elevated temperature for extended times. The development of physical contamination on its surface through the process of diffusion and crystallisation by cyclic oligomers is undesirable and can be immediately and conveniently recognised and measured as the appearance of haze on the film. There is a need to provide an improved barrier to the diffusion of cyclic oligomer from polyester film substrates which are exposed to conditions of elevated temperature for extended times in this technology.
The object of the present invention is to minimise or prevent the formation of haze in a polyester film substrate during the thermal processing thereof at elevated temperatures, for instance during the manufacture of electronic, photonic and optical assemblies or structures. It is a further object of this invention to minimise or prevent the formation of haze in a polyester film which has excellent dimensional stability.